Polonius seems like he is only a meddling idiot who constantly noses about in other people's affairs; he might seem relatively harmless. However, don't discount the impact that all of that meddling has on the lives of those around him. The first area that he meddles in is in prompting Ophelia to break up with Hamlet. Polonius warns Ophelia that even though she thinks Hamlet loves her and is honorable, he indeed isn't, and all he wants is to hook up with her then abandon her. Opehlia, at her father's prompting, agrees to stop seeing Hamlet. This rejection of Hamlet makes Hamlet, who is already mad at his mother's seeming fickleness, even more mad at women, and more incensed at the situation with his uncle and mom. It grows his anger, making him lash out in rage against his mother in her chambers later on.
Polonius also enhances and exaggerates Hamlet's suspicious behavior, drawing attention to it, which makes the king suspicious of Hamlet's intentions--this leads the king to plot against Hamlet. If Polonius hadn't meddled, and pointed out Hamlet's strange behavior to the king, Hamlet might have slipped under the radar and been able to enact his revenge without suspicion.
Then, because of Polonius's meddling in the queen's chamber, he ends up getting killed--because of his death, Hamlet is sent away, giving the king a chance to kill him in secret, away from the limelight. It doesn't work, but, Hamlet's banishment from the kingdom for a time helps Hamlet to realize his own strength, find his will, and decide, once and for all, to enact revenge.
So even though Polonius may seem like a minor character who is a harmless and bumbling old man, he actually does have quite an impact on the events of the story. And does he deserve being murdered in cold blood? I don't think so. We all meddle in other people's affairs; we're all gossips. If anyone who every meddled or gossiped were killed, it would be a quick way to dramatically reduce the human population, don't you think? I hope that helped; good luck!